The invention relates to a method of demodulating a frequency-modulated periodic input signal by determining the duration of each half-cycle between every two adjacent zero crossings of the input signal, which signal appears in the form of a sequence of digitized samples taken from the input signal at regular intervals, and by generating the reciprocals of the duration of the half-cycles. The invention also relates to an arrangement for carrying out the method.
A similar method for input signals in purely analog form is known in which the duration of each half-cycle between every two adjacent zero crossings is determined by counting a reference clock signal whose frequency is high in comparison with the highest frequency of the input signal. The number of reference-clock pulses counted between two adjacent zero crossings constitutes a digital representation of the duration of the half-cycle. If the half-cycle duration must be determined with a high resolution, the frequency of the reference clock signal must be increased proportionally relative to the input-signal frequency. In the case of high-frequency input signals, such as the frequency-modulated luminance signal in a color video signal read from a magnetic tape, the reference clock frequency must be so high that the counting circuits required for this are impracticable or become too intricate and expensive.